1. Field of the Invention
This invention relates to an electroluminescence element which is used as a light source to which a photosensitive element is exposed in an electronic printing apparatus and an electronic printing apparatus employing the electroluminescence element, and more particularly to an electroluminescence device with a reduced delay from the end of the desired light emission period until the complete end of light emission, thus achieving a shorter overall light emission time, and also to an electronic printing apparatus which achieves high speed printing employing the electroluminescence device.
2. Description of the Prior Art
It is already known, as disclosed for example in Japanese Patent Laid-Open Application No. 62-106479 (1987) to employ an electroluminescence element as a light source for an electronic printing apparatus, that is, as a light source to which a photosensitive member is exposed.
FIG. 3 shows the variation in intensity of light emitted from an electroluminescence element in an electronic printing apparatus with respect to time when the electroluminescence element is energized. The time variation of the intensity of light emitted from an electroluminescence element in a conventional electronic printing apparatus will be described subsequently with reference to FIG. 3.
First, normally an AC drive signal is applied to an electroluminescence element, and the waveform (b) in FIG. 3 illustrates such a drive signal applied to the electroluminescence element. The drive signal is applied to the electroluminescence element for a period of time (up to time T1 in FIG. 3) which depends upon an exposure characteristics and so forth of the photosensitive member. Generally, an electroluminescence element has characteristics such that, when a drive signal as described above is applied to the electroluminescence element, the intensity of the electroluminescence element rises gradually and the electroluminescence element reaches its maximum intensity immediately before the application of the driving signal is stopped, as seen from the curve (a) of FIG. 3. Then, after the time T1, the brightness of the electroluminescence element decays gradually as shown by the solid line portion of the curve (a) of FIG. 3 from the time T1 to time T2 and a broken line portion from the time T2.
The demand for an electronic printing apparatus of a higher printing speed has been and is progressively increasing in recent years. In order to raise the printing speed, it is necessary on one hand to increase the intensity of the electroluminescence element and on the other hand to decrease the decay time of the electroluminescence element after emission of light, that is, to cause the intensity of the electroluminescence element to decrease as quickly as possible after the drive signal to the electroluminescence element is cut off, so as to eliminate unnecessary exposure of the photosensitive element to light.
An increase of the intensity of the electroluminescence element can be achieved comparatively easily by adjustment of the concentration of light emitting ions, selection of the dielectric material and so forth. There is no practical obstacle to adjustment of the light emitting ion concentration by these means.
On the other hand, it is known that the light emission decay time can be adjusted by changing the concentration of doping ions in the material forming the electroluminescence element which form the light-emitting centers. For example, when manganese (Mn) is used as a dopant at a concentration of 0.1% by weight in zinc sulfide (ZnS) which is commonly used as a material for an electroluminescence element, the light emission decay time of the electroluminescence element at room temperature is about 1 ms. Further, it is known that, when the concentration of manganese (Mn) is 1% by weight, the light emission decay time is about 0.3 ms. While the light emission decay time is directly related to proportion to the amount of manganese (Mn) dopant in this manner, it is known that, when ZnS is doped with Mn as described above, the light emission efficiency of the electroluminescence element reaches a maximum when the concentration of Mn is about 0.5% by weight, and as the doped amount of Mn increases, a phenomenon called concentration quenching occurs, and instead the light emission efficiency suddenly drops.
Accordingly, reduction of the light emission decay time and enhancement of the light emission efficiency of an electroluminescence element are conventionally contradictory demands, and actually, there is the problem that either one of the light emission decay time and the light emission efficiency must be sacrificed. When such an electroluminescence element is employed for an electronic printing apparatus, it imposes a limit to the printing speed, and particularly when the electroluminescence element is driven, for example, by an AC signal of 20 kHz, the printing time for a line of pixels is not less than 1 ms at the shortest. Accordingly, there is the problem that a medium or high speed electronic printing apparatus cannot be realized with an electroluminescence element.